The field of the disclosure relates generally to electric power systems, and, more particularly, to electric power distribution systems including transformers with tap changers and their methods of operation.
At least some known electric power systems include electric power transformers configured to regulate voltages through the use of on-load tap changers. An on-load tap changing (OLTC) transformer has several connection points, so called “taps”, along at least one of its windings. With each of these tap positions a certain number of turns is selected. Since the output voltage of the OLTC transformer is determined by the turns ratio of the primary windings versus the secondary windings, the output voltage can be varied by selecting different taps. The tap position to connect to is determined by a suitable controller and tap selection is shifted through an on-load tap changing device. Since high voltages are involved, and the taps are changed while the OLTC transformer is under load, each time a tap is changed, arcing occurs. Arcing facilitates deterioration of the associated materials, thereby tending to decrease the service life of the tap changer mechanisms. Therefore, it is typically desirable to shift taps as infrequently as possible. However, it is not unusual to have dozens of tap changes over a 24-hour period, especially with the increasing share of variable and intermittent distributed generation (DG) in the electric power system. The operators of the electric power system determine the tradeoff between the frequency and number of on-load tap changes with the subsequent wear on the tap changer and the quality of the voltage on the portion of the system maintained by the affected OLTC transformer.
Many known on-load tap changer controllers are configured to move the tap in an OLTC transformer automatically as a function of “raise” and “lower” commands to maintain the system voltage at a predetermined value, i.e., a constant voltage set-point. Typically, on-load tap changer controllers monitor the difference between the measured voltage at the on-load tap changer and the voltage set-point. When the difference between this measured voltage and the predetermined voltage set-point exceeds a previously defined tolerance band, a tap change is triggered.
Many known electric power systems include a growing share of distributed generation (DG). Many types of DG significantly increase the variability of the voltage on the portion of the system maintained by the affected OLTC transformer, thereby increasing the frequency of commanded tap changes. Moreover, with a significant portion of DG on one side of the transformer, i.e., typically the lower voltage downstream side, electric power flow through the OLTC transformer may be reversed, i.e., transmitted from the low voltage side to the high voltage side of the transformer. As such, the affected OLTC controller needs to be configured to detect such a power flow reversal and still be able to ensure correct voltage regulation. More specifically, a variable voltage set-point may be necessary. At times of large reverse power flow, which results in high network voltages, especially when DG is connected at remote feeder ends, a low voltage set-point is required. In contrast, during times of high demand by the loads and low network voltages, a higher voltage set-point is desired.
FIG. 1 is a graphical representation of a prior art control scheme 20 for OLTC devices (not shown). Control scheme 20 includes a y-axis 22 that represents a voltage set-point (Vset) for a voltage measured proximate a transformer (not shown) or proximate a feeder end through potential transformers (PTs) (not shown). Y-axis 22 is labeled using the “per-unit” system. Control scheme 20 also includes an x-axis 24 that represents electric power transmitted through the transformer and intersects y-axis 22 at a point 26. X-axis 24 is labeled in kiloWatts (kW) and includes a positive power flow portion 28 to the right of intersection point 26 and a reverse power flow portion 30 to the left of intersection point 26.
Control scheme 20 shows a first known control curve 40 that includes a first segment 42 extending from a maximum reverse power flow value 44 to a first predetermined breakpoint 46 associated with a predetermined reverse power flow value 48 at a constant voltage set-point value of approximately 0.975 pu. Control curve 40 also includes a second segment 50 extending from a maximum forward power flow value 52 to a second predetermined breakpoint 54 associated with a predetermined forward power flow value 56 at a constant voltage set-point value of approximately 1.025 pu. Control curve 40 further includes a third segment 58 that extends linearly from first breakpoint 46 to second breakpoint 54 through an intersection with y-axis 22.
Control curves 40 provides a variable voltage set-point as a function of forward and reverse power flow. Since the measured voltage proximate the on-load tap changer is virtually constant, the difference between the measured voltage and the voltage set-point will change by a variation of the voltage set-point value. As described above, a tap change is induced when this difference exceeds the predefined tolerance band. Therefore, a tap change is essentially triggered by varying the voltage set-point. In accordance with this, many tap changes can be expected for linear segment 58 of curve 40. Moreover, since the overall power flow dependent voltage set-point characteristic is substantially static, many tap changes may be experienced throughout a normal cycle of power flow through the transformer due to control curve 40.